1. Field of the Invention
This invention relates to an electrostatographic imaging method and a photoconductive composition useful therein. More specifically, this invention provides a photoconductive composition wherein the non-persistent photocurrent is enhanced by sensitization with small concentrations of a protonic acid. Such composition is highly photosensitive and capable of rapid cycling without fatigue when imaged in accord with the method of this invention.
2. Description of the Prior Art
The formation and development of images on the imaging surfaces of photoconductive materials by electrostatic means is well known. The best known of the commercial processes, more commonly known as xerography, involves forming a latent electrostatic image on an imaging surface of an imaging member by first uniformly electrostatically charging this imaging surface and then exposing this electrostatically charged surface to a light and shadow image. The light struck areas of the imaging surface are thus rendered conductive and the electrostatic charge selectively dissipated in these irradiated areas. After the photoconductor is exposed, the latent electrostatic image on this image bearing surface is rendered visible by development with a finely divided colored electroscopic material, known in the art as "toner". This toner will be principally attracted to those areas on the image bearing surface which retain the electrostatic charge and thus render visible the latent image.
The developed image can then be read or permanently affixed to the photoconductor where the imaging surface is not to be reused. This latter practice is usually followed with respect to the binder type photoconductive films (e.g. ZnO) where the photoconductive imaging layer is also an integral part of the finished copy.
In so-called "plain paper" copying systems, the latent image can be developed on a reusable photoconductive surface or transferred to another surface, such as a sheet of paper, and thereafter developed. When the latent image is developed on a reusable photoconductive surface, it is subsequently transferred to another substrate and then permanently affixed thereto. Any one of a variety of well-known techniques can be used to permanently affix the toner image to the copy sheet, including overcoating with transparent films, and solvent or thermal fusion of the toner particles to the supportive substrate.
In the above "plain paper" copying system, the materials used in the photoconductive layer should preferably be capable of rapid switching from insulative to conductive to insulative state in order to permit cyclic use of the imaging surface. The failure of a material to return to its relatively insulative state prior to the succeeding charging sequence will result in an increase in the dark decay rate of the photoconductor. This phenomenon, commonly referred to in the art as fatigue, has in the past been avoided by the selection of photoconductive materials possessing rapid switching capacity. Typical of the materials suitable for use in such a rapidly cycling system include anthracene, sulfur, selenium and mixtures thereof (U.S. Pat. No. 2,297,691); selenium being preferred because of its superior photosensitivity.
In addition to anthracene, other organic photoconductive materials, most notably, poly (N-vinylcarbazole), have been the focus of increasing interest in electrophotography. Most organic photoconductive materials, including poly(N-vinylcarbazole), lack the inherent photosensitivity to be competitive with selenium. This need for the enhancement of the photoresponse characteristics of organic photoconductors thus led to the formulation of these organic materials with other compounds, commonly referred to as "activators". Poly (vinylcarbazoles), for example, when sensitized with 2,4,7,-trinitro-9-fluorenone exhibit good photoresponse and discharge characteristics and, (depending upon the polarity of the surface charge), low dark decay; U.S. Pat. No. 3,484,237. Other organic resins, traditionally considered nonphotoconductive can also be sensitized with certain activators, such as Lewis Acids, thus forming charge transfer complexes which are photoresponsive in the visible band of the spectrum U.S. Pat. Nos. 3,408,181; 3,408,182; 3,408,183; 3,408,184; 3,408,185; 3,408,186; 3,408,187; 3,408,188; 3,408,189; and 3,408,190. With respect to both the photoconductive and nonphotoconductive resins, the degree of sensitization is generally concentration dependant; the higher the loadings of activators, the greater the photoresponse.
The concentration of activator capable of formulation with the above materials, however, is finite; generally being limited to less than 10 weight percent of the composition. Ordinarily, the addition of high loadings of activator to many of the above materials will lead to impairment of mechanical and/or the photoconductive properties of the sensitized composition. In most instances, the excessive addition of activators to both the photoconductive and nonphotoconductive materials of the types disclosed in the above patents will result in crystallization of these activators, thus impairing the mechanical strength and other physical properties of the resultant photoconductive composition. Still yet other sensitizers, when present in relatively low concentrations can result in over sensitization of the composition in that the photocurrent generated upon exposure will persist long after illumination ceases, BUL. CHEM. SOC. of JAP. 39: 1660 - 1670 (1966). This phenomenon, commonly referred to in the art as "fatigue" prevents the further use of such materials for preparation of successive electrostatic reproductions until such persistent conductivity is dissipated in the previously illuminated areas of the photoconductor. This dissipation of persistant photocurrents generally takes an extended period of time and/or thermal erasure, thus making these oversensitized compositions generally unsatisfactory for rapid cycling electrostatographic imaging systems.
It is therefore the object of this invention to provide an acid sensitized photoconductive composition useful in a rapidly cycling electrostatographic imaging process.
Another object of this invention is to provide an imaging system wherein the photoconductive materials are highly photosensitive as a result of the enhancement of the non-persistent photocurrents.
Another of the objects of this invention is to provide an imaging system wherein enhancement of non-persistent photocurrents is the result of sensitization of a photoconductive charge transfer complex with an acid sensitizer.